This invention relates to sintered bodies of aluminum nitride, more particularly, sintered bodies of aluminum nitride which have high density and are excellent in various properties for practical use.
Sintered bodies of aluminum nitride (AlN) have been noted as starting materials having properties essential to high-temperature materials, such as heat resistance, corrosion resistance or high-temperature impact resistance, and also as materials having high thermal conductivity.
In the meantime, it is required that sintered bodies of AlN have high density in order to allow them to exhibit these various properties. For this reason, many efforts have hitherto been made to establish the technique for preparing sintered bodies of AlN having high density.
Such sintered bodies of AlN are usually obtained by molding AlN powder, followed by sintering. However, in the process in which AlN powder is used alone, high density sintered bodies can not be obtained due to poor sinterability thereof, and the density of the sintered bodies is considerably low, i.e., at most 82% of the absolute density.
For this reason, an attempt to utilize a method of sintering under pressure in which a hot pressing is employed has been made but a satisfactory result has not been obtained. Moreover, a method has been tried in which rare earth elements' oxides such as yttrium oxides (Y.sub.2 O.sub.3), lanthanum oxides (La.sub.2 O.sub.3) or the like are added to AlN powder as sintering aids to obtain sintered bodies of considerably higher density.
However, this method has disadvantages in that it has difficulties in view of cost, due to the expensiveness of rare earth oxides, and that conductivity of AlN-Y.sub.2 O.sub.3 series sintered bodies is liable to be lower so that high thermal conductivity inherent to AlN can not be maintained and exhibited.
There is disclosed in Japanese Provisional Patent Publication No. 23411/1975, a process for preparing sintered bodies of AlN in which calcium oxide (CaO), barium oxide (BaO), strontium oxide (SrO) or the like is added as a sintering aid. It was found that these sintering aids are effective additives for imparting to the sintered bodies characteristics inherent to AlN since a small amount of such a sintering aid forms a liquid phase at a high temperature, which is advantageous for the densification of sintered bodies of AlN.
On the other hand, it was found that AlN powder, which is a main starting material for sintered bodies of AlN, is liable to contain oxygen or oxides, unavoidably mixed at the time of its preparation, and they produce a harmful effect on maintaining the excellent properties of sintered bodies of AlN. Namely, oxygen is liable to dissolve in AlN powder to form a solid solution so that it often exists not only on the surface of AlN powder but also within the inner part of the powder. Moreover, oxygen may also be incorporated in the form of Al.sub.2 O.sub.3 during the preparation of AlN. Accordingly, when sintering of AlN powder is carried out at high temperature, oxygen remains in the state of a solid solution or is converted to a compound having, for example, a `spinel type structure` defined by the formula (AlN).sub.x (Al.sub.2 O.sub.3).sub.y (wherein x and y individually represent a given natural number) and so on, resulting in impairing the excellent properties such as high thermal conductivity and the like, which are inherent to AlN. Therefore, in the conventional sintered bodies of AlN or the process for preparing thereof, above-mentioned problems have not been solved at all.